This is a slice of the midline vermis in the cerebellum of a 60-day-old rat that was injected with 3H-thymidine on 4 consecutive days: P6 through P9.
Nearly all cells in the GRANULAR LAYER are still heavily labeled in all lobules. An occasional granule cell is unlabeled in the lingula, centralis, nodulus and in the deep parts of the other lobules; these are the oldest granule cells. Between P4 and P6, the external germinal layer is still producing only a few granule cells--most cell proliferation is just increasing the numbers of granule cell precursors in the external germinal layer. Unlabeled cells in the lower half of the molecular layer are now commonly seen in the lingula, centralis, and nodulus. These are basket cells that were generated before P6. A fair number of unlabeled cells are in the upper half of the molecular layer in these same lobules; these are the oldest stellate cells. Thus, by the morning of P6, neurogenesis is rapidly continuing in the basket cell population, is already well on the way in the stellate cell population, and is off to a slow start in the granule cell population. The composition of the external germinal layer is predominantly granule cell precursors. The external germinal layer also contains most stellate cell precursors, but fewer basket cell precursors are there, especially in the lobules at the base of the cerebellum and in parts of the other lobules that are nearer the core of the cerebellum.
Most likely composition of the external germinal layer (egl) at P6:
1. The egl is rapidly proliferating and thickening due to the generation of precursor cells for granule cells; it is still "stockbuilding" these precursors.
2. This is the time of peak numbers of stellate cell precursors; many are entering their final neurogenetic divisions.
3. Many precursors of basket cells already have entered their final neurogenetic division, and the number of basket cell precursors is rapidly declining as these neurons are being generated.